A good pro should but this is the generation of we want it now, 1 club players aren't too common these days
Rashford, as good as he has been is a little too one-dimensional, his speed coupled with space is his best asset, without space he tends to struggle, he's not very good at holding up the ball and linking play that way, unless he can gain those skills he's essentially a LW, we already know he's not effective on the right.
Garnacho is potentially a generational talent, with more experience he will gain all the attributes of Rashford, he's also several years younger, he could probably be as effective on the RW as the LW in which case having both isn't an issue, but if he's seen as a LW then he's vying for the same position, in that scenario I see one of them going